I don't really understand what you're saying here.
From what I understand, the quoted statement is that BMW have provided the required bracing with the standard diagonal brace set up. We know the chassis is already very stiff.
So I don't know the answer to this but I'm reading into this that BMW M Engineers felt the level of bracing was correct for the cars application. Now of course you can continue to stiffen, but the question would be would that improve things or actually detract from the handling set up?
I would think a strut brace would be a low cost item for BMW to fit if they thought they needed it, but then would it the case that the standard set up is fine for road use but if you are tracking the car regularly or want the extra bracing to sharpen the front even more at the expense of some compliance then you add a brace aftermarket etc?